1. Field of the Invention
The present invention generally relates to an electrophotographic image forming apparatus such as a copying machine and, more particularly, to a developing device utilized in the electrophotographic image forming apparatus for developing a powder image.
2. Description of the Prior Art
A developing device is known which comprises a developer tank accommodating therein a mass of developing material, a developing sleeve having a magnetic roller and supported within the developer tank with a portion of the outer peripheral surface thereof exposed towards a photosensitive drum through a rectangular opening defined in the wall of the developer tank. This known developing device is so designed and so operable that the mass of the developing material mixed within the developer tank can be supplied onto the outer peripheral surface of the developing sleeve to form a magnetic brush which is subsequently utilized to develop an electrostatic latent image, formed on the outer peripheral surface of the photosensitive drum, into a toner image.
In this prior art developing device, in order to avoid any possible fall of particles of the developing material out from a gap defined in the vicinity of the rectangular opening between an inner surface of the developer tank and the developing sleeve, the gap defined between the inner surface of the developer tank and the outer peripheral surface of the developing sleeve is limited to a predetermined size, for example, 1.5 mm.
In the case of a compact, medium or low speed copying machine wherein the developing sleeve has a relatively small diameter and also has a small number of magnetic poles, the magnetic poles within the developing sleeve are carefully positioned to permit the developing sleeve to exert magnetic forces of attraction thereby to minimize separation from the developing sleeve the particles of the developing material which have been attracted onto the developing sleeve.
In this prior art developing device, it has, however, been found that the gap between the inner surface of the developer tank and the developing sleeve tends to be narrowed when particles of the developing material adhere to the inner surface of the developer tank. Once this occurs, some of particles of the developing material often fall out from the rectangular opening and contaminate some component parts of the copying machine disposed beneath the rectangular opening, possibly because an increased resistance acts on magnetic brush bristles then revolving in sliding contact with the inner surface of the developer tank. Depending on the design and location of the developing device within the machine, the developing particles falling down may contaminate copying papers and will constitute a cause of reduction in copy quality.